This invention generally relates to the field of communication systems and, more particularly, to digital communication systems.
Some digital communication systems known as time division multiple access (TDMA) subdivide one or more radio frequency (RF) channels into a number of time slots during which mobile units within the system engage in digital voice and data communication. In these systems, the mobile units engage in communication through a plurality of scattered base stations, by transmitting and receiving bursts of digital information during allocated transmit and receive time slots. Existing TDMA systems utilize a number of access techniques that support duplex communication. For duplex communication, a TDMA/FDD communication system, such as one based on the Global System for Mobile (GSM) communication standard, uses separate receive and transmit RF channels.
Digital cellular radio telephone systems, which divide a geographical area up into cells, widely use TDMA communication systems for providing wireless communication among subscribers of mobile units and telephone units that are connected to a public switched telephone network (PSTN). Within each cell, a base station communicates with the mobile units over uplink and downlink RF channels. The base station transmits bursts of information to the mobile units over the downlink RF channels, and the mobile units transmit such bursts to the base stations over the uplink RF channels.
To avoid interference, neighboring cells are generally allocated different RF channels. Because of the relatively low power RF transmissions within a particular cell, another cell spaced two or more cells apart may typically reuse the same frequency. The farther the cells reusing the same frequencies are from each other, the lower the interference level between them. Therefore, for maintaining good quality RF communication links between the base stations and the mobile units, the frequency reuse cell pattern is an important factor in achieving a desired carrier-to-interference (C/I) ratio in a cell.
During the initial phase of a network, it is important to provide wide coverage using a smaller number of cells. With the increase in system capacity, it becomes necessary to increase the number of cells and reduce their size. Under this arrangement, it is essential to provide the RF link using a minimum amount of radiated power between the mobile units and the base station. To provide the uplink RF channels with minimized radiated power, it is customary to equip the base stations with low noise amplifiers, which are positioned at close proximity to the antennas of the base station. This arrangement improves the receiver sensitivity for the signals received from the mobile units. On the other hand, for the downlink RF channel, some conventional approaches rely on high power transmitters at the base station or booster amplifiers, which are positioned on the ground or on the mast of base station antennas. In addition to complicating operation and maintenance of the system, these approaches suffer from other serious drawbacks, including excessive heat generation at the base station and introduction of spurious noise.
Another conventional approach for extending communication coverage over downlink RF channels employs multicasting techniques. Multicasting techniques are widely used in analog communication systems for paging and trunked communication. Multicasting is especially beneficial for reaching mobile units that are positioned at the fringes of a communication coverage area. Most conventional multicasting techniques employ two or more separated antennas at each base station, to transmit the same messages over the same coverage area. The messages are multicasted either simultaneously or with some offset in time. In TDMA communication systems, the transmission times from the separated antennas are offset in order to introduce time diversity as well as space diversity to the multicast transmissions. The transmissions are offset in time by one or more symbol times, which are the durations of a corresponding number of data bits. Under well known theories, multicasting using antenna diversity and time diversity in TDMA systems improves coverage and communication quality. Some of these theories are disclosed in the following publications: Artificial Delay Insertion Diversity To Extend Anti-Multipath Capability Of DSK In Mobile Radio, Susumu Yoshida, Fumio Ikegami, Tsutomu Takeuchi, Sirikist Ariyavisitakul, and Masaaki Sasada, IEEE in 1986; Combined Space/Time Diversity Technique For Narrowband TDMA Mobile Systems, L. B. Lopes, Dept. of Electrical & Electronic Engineering, University of Leeds, May 19, 1989, Electronic Letters, Jul. 20, 1989; GSM Base-Station Antenna Diversity Using Soft Decision Combining On Up-Link and Delayed-Signal Transmission on Down-link, Preben E. Mogensen, Danish Center For Personal Communication, Aalborg University, IEEE 1993; and On Antenna- And Frequency Diversity In GSM Related Systems (GSM-900, DCS-1800, and PCS1900), Preben E. Mogensen and Jeroen Wigard, Danish Center for Personal Communication, Aalborg University, IEEE 1996.
While providing benefits, multicasting uses more of the valuable communication resources. In TDMA systems, for example, a number of communication time slots must be reserved for multicasting. Downlink multicasting in such TDMA systems uses a predefined number of time symbols for offsetting the transmission times during a number of reserved time slots. By multicasting during reserved time slots, valuable channel resources may be wasted, if multicasting on a particular link is not necessary. Furthermore, multicasting during reserved time slots increases interference within the system, especially the interference on channels that are reused in other cells. Therefore, there exists a need for a TDMA communication system that improves downlink communication coverage by multicasting on downlink RF channels without wasting communication resources unnecessarily.